A resistance change element using metal ion transfer and electrochemical reaction in a resistance change film includes three layers of a copper electrode, a resistance change film, and an inert electrode. The copper electrode serves not only as an electrode, but also as a member to supply metal ion to the resistance change film. A material of the inert electrode is a metal which does not supply metal ion to the resistance change film. The term inert electrode means an electrode that does not contribute to reaction. When the copper electrode is grounded and a negative voltage is applied to the inert electrode, a metal of the copper electrode is converted into metal ion and is dissolved in the resistance change film. Then, the metal ion in the resistance change film is precipitated as a metal in the resistance change film and the precipitated metal forms a metal-bridge that connects the copper electrode and the inert electrode. By electrically connecting the copper electrode and the inert electrode with the metal-bridge, state of the resistance change element is translated from a high-resistive state to a low-resistive state.
On the other hand, when the copper electrode of the resistance change element in the low-resistive state described above is grounded and a positive voltage is applied to the inert electrode, the metal-bridge is dissolved in the resistance change film, and part of the metal-bridge is broken. Accordingly, electric connection between the copper electrode and the inert electrode by the metal-bridge is broken, and thus state of the resistance change element returns to the high-resistant state. Electric characteristics of the resistance change element changes such that the resistance between the copper electrode and the inert electrode increases or an inter-electrode capacitance varies, before a stage of the electric connection is completely broken. Finally, the electric connection therebetween is broken. The high-resistive state described above can be changed to the low-resistive state again by applying negative voltage to the inert electrode.
NPL 1 proposes a changeover switch in a programmable device using the resistance change element. By using the resistance change element, a switch area can be reduced to 1/30 that of switches of other types, and a switching resistance can be reduced to 1/40. Moreover, the resistance change element may be integrated into a interconnect layer. Therefore, reduction in chip area and improvement of signal delay are expected.
PTL 1 and PTL 2 disclose methods of manufacturing the resistance change element in an integrated circuit.
PTL 1 discloses a method of integrating a resistance change element in a copper multilayer interconnect. According to PTL 1, one copper line out of the copper interconnect forms a copper electrode of the resistance change element, so that the copper interconnect works also as the copper electrode of the resistance change element. The configuration realize density increasing of the elements by miniaturizing the resistance change elements and manufacturing process simplifying. The resistance change elements can be mounted only by adding two photomasks to a normal copper damascene interconnect process. Consequently, it reduces the cost simultaneously. Further, it can improve the performance of an apparatus by mounting the resistance change element in a leading-edge device composed of the copper interconnect.
According to FIG. 3 of PTL 1, an opening portion that communicates with part of a first interconnect is formed by dry-etching an insulating barrier film, and resistance change element films are deposited so as to cover the exposed first interconnect. Subsequently, a configuration of the resistance change element is made by forming a first upper electrode and a second upper electrode.
PTL 2 also discloses a method of integrating a resistance change element in a copper multilayer interconnect. In FIG. 17 of PTL 2, an opening portion is provided in an insulating barrier film to expose parts of upper surfaces of the copper interconnect (first interconnects 5a, 5b), and a resistance change element film, a first upper electrode and a second upper electrode are formed on the copper interconnect. Here, the opening portion is provided to expose one end of each of the copper interconnects, and the ends contact with the resistance change element film.
FIG. 12 illustrates a cross-sectional structure of a resistance change element disclosed in FIG. 11 of PTL 2. A first resistance change element includes the first copper interconnect 5a′, a resistance change film 9′, and an upper electrode 10′. A second resistance change element includes a first copper interconnect 5b′, the resistance change film 9′, and the upper electrode 10′. The first copper interconnects 5a′, 5b′ are covered with barrier metals 6a′, 6b′ except for upper surfaces thereof and are embedded in a first interlayer insulating film 4′. The upper surfaces of the first copper interconnects 5a′, 5b′ are covered with a first barrier insulating film 7′, and are in contact with the resistance change film 9′ via an opening portion 26′ (illustrated in FIG. 13) provided in the first barrier insulating film 7′.
The resistance change film 9′ covers the opening portion 26′ of the first barrier insulating film 7′ and is partly in contact with an upper surface of the first barrier insulating film 7′. The resistance change film 9′ is in contact with the upper electrode 10′. The upper electrode 10′ is in contact with a copper-plug 19′ covered with a barrier metal 20′ on a surface thereof. The plug 19′ is in contact with a second copper interconnect 18′. The plug 19′ and the second copper interconnect 18′ are embedded in a second interlayer insulating film 15′, and an upper surface of the second copper interconnect 18′ is covered with a second barrier insulating film 21′.
FIG. 13 illustrates a cross-sectional view and a plane view of a step of opening the first barrier insulating film 7′ for manufacturing a structure illustrated in FIG. 12. In the step of forming the opening portion 26′, a contact area between the resistance change film 9′ and the first copper interconnect 5a′ is preferable equivalent to a contact area between the resistance change film 9′ and the first copper interconnect 5b′.
Electric characteristics of a device with the structure in FIG. 12 and a photograph of the opening portion are disclosed in NPL 2. According to the electric characteristics disclosed in NPL 2, two sets of resistance change elements are referred to as a complementary resistance change element (complementary atom switch, CAS), and high OFF-state reliability is achieved while reducing a program voltage. The program voltage is a voltage when the resistance of the resistance change element changes from the high-resistive state to the low-resistive state, and is preferably not higher than 2V. In the case where the resistance change element is applied to a programmable logic described in NPL1, the resistance is required not to vary even when an operation voltage (1V, for example) of the integrated circuit is applied. In other words, the high OFF-state reliability is required, such that ensures no variation of high-resistive state to the low-resistive state occur even when a voltage of 1V continuously applied to the element for 10 years. The 1V corresponds to the operation voltage of an integrated circuit and 10 years corresponds to a life of the integrated circuit. The complementary atom switch solves the subject described above by the following method.
The metal deposition type resistance change element has a bipolar feature. The following is a description of a case where two resistance change elements in a high-resistive state are connected in series in an opposite direction and a voltage is applied to both ends. Here, the term “connected in series in the opposite direction” indicates connecting two inert electrodes of two resistance change elements or two copper electrodes of two resistance change elements. In FIG. 12, the upper electrode 10′, which corresponds to the inert electrode is shared, that is connected. When a voltage is applied between both ends, that is, the first copper interconnect 5a′ and the second copper interconnect 5b′, a voltage of a polarity which does not cause a resistance change is applied to one of the two resistance change elements irrespective of polarity of the voltage. In this configuration, it is reported that the high-resistive state may be maintained for 10 years or more even when applying 1V, which is the operation voltage of the integrated circuit (FIG. 16 in NPL 2).
It is also reported that the resistance varies by a low voltage, about 2V, by applying a voltage independently to each of the resistance change elements while programming the elements connected in series (FIG. 9(a) in NPL2). Contact of the end portions of the first copper interconnect 5a′ and the first copper interconnect 5b′ to the resistance change film 9′ also contributes to reduction of a program voltage. The program voltage of the structure illustrated in FIG. 12, in which the resistance change film is in contact with the end portions, is lower than that of the structure in PTL 1 (FIG. 1 in PTL1), in which the resistance change film in contact with a flat portion of the copper interconnect. At the end portions of the copper interconnect, the shape of the copper is pointed. When the end of the electrode is pointed, concentration of electric field may occur. In other words, the electric field is intensified by the structure having the pointed end, generation or transfer of copper ion is activated, and a low program voltage is realized.
Techniques relating to the resistance change elements and semiconductor devices employing the resistance change elements are also disclosed in PTL 3, PTL 4, and PTL 5.